The present invention relates to a drive for transmitting motion between a drive shaft and a pair of driven shafts, in particular, for controlling the front and rear drive shafts of a motor vehicle.
Known drives of the aforementioned type usually comprise a power dividing unit located between the drive shaft and the driven shafts and designed to transmit the torque on the drive shaft to each driven shaft in such a manner that the torques acting on each of the same fall within a given torque ratio and such as to control the two driven shafts at speeds depending on the operating conditions of the members connected to the shafts themselves.
On automobile drives, the said power divider usually consists of an epicyclic train comprising a pair of coaxial sun gears, each integral with one of the said driven shafts, and between which are located planet gears turning on the pins of a train carrier controlled by the drive shaft.
A train of the aforementioned type is known to divide the torques acting on the two driven shafts according to a constant ratio, regardless of the speeds of the shafts themselves, and depending solely on the ratio between the pitch circle radii of the said two sun gears. Furthermore, such a train permits the said driven shafts to operate at different speeds, depending on the speeds imposed on the vehicle wheels under various driving conditions, e.g. when cornering.
On drives of the aforementioned type, in the event of a zero resisting torque on one of the driven shafts, as occurs in the case of maximum slip on one of the drive axles on the vehicle (e.g. when driving over icy road surfaces), the speed of the other driven shaft is also zeroed with the result that effective tangential forces can no longer be transmitted between the wheels and the road surface.
To overcome this drawback, drives of the aforementioned type are fitted with special couplings designed to connect the two driven shafts when the relative speed of the same is other than zero. Couplings of this type substantially comprise a cylindrical casing housing two sets of discs, each connected to a respective driven shaft, with each disc in one set located between a pair of discs in the other. The discs are provided with appropriate perforations and slots, and the casing filled with a special silicone fluid so that, in the event of relative rotation of the discs in the said two sets, resulting from a difference in the speed of the two shafts, the said fluid is drawn out and heated, with the result that its physical characteristics, in particular viscosity, are altered. Consequently, the torque transmittable by the coupling increases alongside an increase in the difference between the speeds of the two driven shafts, so as to transmit the torque on the rotary driven shaft to the fixed one, and from the wheels to the road surface, and so restore normal road holding conditions.
Drives of the aforementioned type present a number of drawbacks.
Firstly, torque transmission from one to the other of the said driven shafts is fairly sudden, thus resulting in off-balancing and possible swerving of the vehicle. The reason for this is that the relationship between the torque transmitted by the coupling and the speed of the drive shaft controlling rotation of the same presents, throughout, a derivative which decreaess alongside an increase in the said speed of the shaft. Furthermore, should the coupling be activated for more than a few tens of seconds, it may actually lock, with the result that the two driven shafts are locked integral with each other, which situation may lead to serious off-balancing and swerving of the vehicle. The reason for this lies in the increase in the pressure of the silicone fluid caused both by heating during operation and the fact that the casing on the coupling is filled almost to maximum capacity.